A mineral is a naturally occurring solid with an ordered atomic structure and a definite—but not necessarily fixed—chemical composition. Minerals are generally formed by natural geological processes.
That sounds straightforward.
Then try classifying a few familiar materials.
Quartz? Mineral.
Diamond? Mineral.
A lab-grown diamond? Strictly speaking, no.
Obsidian? Not quite.
Mercury? No.
Ice? Under the right natural conditions, yes.
A pearl? More complicated than you might expect.
The scientific definition of a mineral looks simple because it compresses several different ideas into a single sentence. Pull those ideas apart, and you discover something more interesting: being a mineral depends not only on what a substance is made of, but how its atoms are arranged, how it formed, and even what physical state it is in.
Rocks don't like to be put in boxes. And that is why mineralogists are so particular about the word.
What is a mineral? The short answer
In mineralogy, a mineral is generally defined by four essential characteristics:
-
It occurs naturally.
-
It is a solid.
-
Its atoms or ions have an ordered internal arrangement.
-
It has a definite chemical composition, although that composition may vary within limits.
Minerals are also generally associated with inorganic processes, although nature supplies some fascinating complications there too.
Each part of the definition does real work.
1. A mineral must occur naturally
This is the rule that separates a natural diamond from a laboratory-grown diamond.
A natural diamond forms through natural processes within Earth.
A lab-grown diamond is manufactured.
Chemically, the two can be essentially the same: both consist predominantly of carbon arranged in the diamond crystal structure. They can have extremely similar hardness, optical behavior, density and other physical properties.
But under the strict mineralogical definition, the lab diamond is a synthetic crystalline material rather than a mineral because it did not form naturally.
The same distinction applies to lab ruby and sapphire.
Natural ruby and sapphire are varieties of the mineral corundum.
Their lab equivalents can have the same corundum structure and essentially the same underlying chemistry, but their artificial origin excludes them from the strict definition of a mineral.
This is an important distinction, but it should not be misunderstood.
Calling a lab gem a material rather than a mineral does not mean the mineral science suddenly stops applying.
Quite the opposite.
Because natural and synthetic versions can share the same chemical composition and crystal structure, the same principles of crystallography, hardness, cleavage, optical behavior and atomic bonding largely apply to both.
The distinction is about origin, not scientific legitimacy.
2. A mineral must be a solid
Minerals have an internal structure in which atoms or ions occupy relatively fixed positions.
That requirement excludes liquids and gases.
So liquid water is not a mineral.
But naturally occurring ice can be.
The chemistry has not changed: both are H₂O. What has changed is the physical state and the arrangement of the molecules.
The same principle explains why native mercury is normally excluded from the mineral definition. Mercury may occur naturally, but at ordinary surface conditions it is a liquid.
Once again, chemistry alone is not enough.
For mineralogists, structure and physical state matter.
3. A mineral has an ordered atomic structure
This may be the most important part of the definition.
In a mineral, atoms or ions are arranged in an orderly three-dimensional framework. That repeating internal organization is what we mean when we describe a substance as crystalline.
The arrangement is microscopic, but its effects can be spectacularly visible.
Crystal shape.
Cleavage.
Hardness.
Density.
Optical properties.
Even the way a mineral breaks can reflect the architecture of atoms inside it.
Consider diamond and graphite.
Both are made of carbon.
Yet diamond is the hardest reference mineral on the Mohs scale, while graphite is extremely soft.
Why?
Because their carbon atoms are arranged and bonded in profoundly different ways.
Same element. Different atomic structure. Different mineral.
This is one of the central lessons of mineral science: knowing which elements are present tells you only part of the story.
You also need to know what those atoms are doing.
So is glass a mineral?
Usually, no.
Natural volcanic glass such as obsidian is naturally occurring, inorganic and solid.
But it lacks the long-range ordered atomic structure expected of a true crystalline mineral.
Its atoms are arranged more irregularly.
For that reason, obsidian is generally classified as a mineraloid rather than a mineral.
A mineraloid is a naturally occurring material that resembles a mineral but does not satisfy every part of the strict mineral definition.
Opal is another famous example commonly discussed in this category because its internal structure differs from that of a conventional crystalline mineral.
This distinction is especially useful in gemology.
Something can be beautiful, natural, valuable and geologically fascinating without technically being a mineral.
4. A mineral has a definite—but not necessarily fixed—chemical composition
This phrase needs unpacking.
A beginner might reasonably imagine that every mineral has one exact chemical formula that never changes.
Some come close.
Quartz is represented by SiO₂.
Diamond is C.
But many minerals permit chemical substitutions within their crystal structures.
One ion can sometimes replace another if its size and electrical charge are sufficiently compatible with the structure.
As a result, the chemistry of a mineral may vary within limits without turning it into an entirely different mineral.
This is why mineralogists say a mineral has a definite but not necessarily fixed composition.
The crystal structure imposes rules.
But those rules may allow some flexibility.
The chemistry can vary without changing the mineral
Dolomite provides a useful example.
Its idealized formula is:
CaMg(CO₃)₂
But in natural dolomite, some of the magnesium may be replaced by chemically compatible elements such as iron or manganese.
The exact chemistry therefore varies from specimen to specimen.
Yet it is still dolomite.
This kind of substitution is extremely common in mineralogy.
It also helps explain one of the great attractions of gemstones: tiny chemical changes can produce dramatic changes in color.
Ruby offers a classic example.
Ruby is red corundum.
The basic mineral is aluminum oxide, Al₂O₃. Small amounts of chromium substituting into the crystal structure produce ruby's characteristic red coloration.
Sapphire is also corundum, but different trace elements and combinations of elements can create blues, yellows, greens and other colors.
The bulk crystal remains corundum.
A tiny amount of chemistry transforms its appearance.
Does a mineral have to have one exact formula?
No.
This is one of the most useful corrections to the simplified definition people often learn at school.
A mineral needs a defined range of composition, not necessarily a composition that is identical atom-for-atom in every specimen.
Minerals can form what geologists call solid solutions, in which different elements substitute for one another while the basic crystal structure remains intact.
So a chemical formula written in a mineral guide is often an ideal representation of the structure rather than a claim that every specimen on Earth has precisely that chemistry.
Nature tolerates variation.
But not unlimited variation.
Eventually, changing the chemistry or structure enough produces a different mineral.
One chemistry can also produce different minerals
Now reverse the problem.
Sometimes minerals can have the same chemical composition but different crystal structures.
These are known as polymorphs.
Diamond and graphite are the famous example: both are carbon, but their internal structures differ radically.
Calcium carbonate provides another.
Calcite and aragonite both have the composition CaCO₃.
Yet their atoms are arranged differently, giving the two minerals different crystal structures and physical properties.
So mineral identity is determined by a combination of:
chemistry + structure
Neither one alone is always sufficient.
What about minerals made by living things?
This is where the apparently neat boundary between geology and biology begins to blur.
The traditional mineral definition emphasizes inorganic natural processes.
But organisms routinely interact with mineral-forming chemistry.
Mollusks produce shells containing crystalline calcium carbonate.
Pearls commonly contain the mineral aragonite.
Bones and teeth contain crystalline calcium-phosphate materials related to apatite.
Some organisms even produce tiny magnetic mineral crystals.
The process by which living things form mineral material is known as biomineralization.
This creates an important distinction.
An entire shell or pearl is not simply one homogeneous mineral crystal. It is a biological composite containing mineral material along with organic components and complex structures.
But minerals occurring within those biological materials can still be genuine mineral phases.
Nature does not always respect the neat disciplinary borders humans draw between geology, chemistry and biology.
Is a rock a mineral?
Usually, no.
A mineral is a substance with a characteristic chemistry and crystal structure.
A rock is generally an aggregate of one or more minerals or mineral-like materials.
Granite, for example, commonly contains several minerals, including quartz, feldspar and mica.
Quartz is a mineral.
Feldspar is a mineral group.
The granite made from them is a rock.
A useful analogy is cooking:
Minerals are ingredients. Rocks are the mixtures made from them.
It is simplified, but it gets the essential distinction right.
Mineral, crystal, rock and gem are not synonyms
These words overlap, but they describe different things.
A mineral is defined by its natural occurrence, solid state, chemistry and ordered atomic structure.
A crystal describes a material whose atoms are arranged in an ordered structure. Crystals can be natural or synthetic.
A rock is an aggregate or mass composed of minerals, mineraloids or other geological materials.
A gem is a material valued primarily for beauty, rarity, durability or use in jewelry.
That means a gem may be a mineral—but it does not have to be.
Natural diamond is both a mineral and a gem.
Ruby is a gem variety of the mineral corundum.
Obsidian can be fashioned as a gem material even though it is not a true mineral.
A lab-grown diamond is a crystalline gem material, but under the strict geological definition it is not a naturally occurring mineral.
The categories answer different questions.
Why are mineralogists so strict about the definition?
Because the definition carries information.
If a geologist calls something a mineral, that single word already tells us something about:
-
how it occurs;
-
its physical state;
-
its internal atomic organization;
-
its chemical composition;
-
and the natural processes associated with its formation.
Scientific definitions are not designed merely to sort things into boxes.
They help scientists communicate efficiently.
They also help us understand why materials behave as they do.
Minerals are records of how planets work
Minerals are far more than attractive crystals.
They are products of geological processes.
The minerals present in a rock can reveal clues about the temperature, pressure and chemical environment under which that rock formed.
Some mineral assemblages tell geologists about processes deep inside Earth.
Others record volcanic activity, hydrothermal alteration, weathering or interaction with water.
Minerals can preserve evidence of conditions that existed millions or billions of years ago.
They occur not only on Earth but throughout the solar system: in meteorites, on the Moon, and on other planetary bodies.
In that sense, mineralogy is not simply the science of identifying pretty crystals.
It is one of the ways we reconstruct the history of worlds.
So what is a mineral?
The useful short definition is:
A mineral is a naturally occurring solid with an ordered atomic structure and a definite, though sometimes variable, chemical composition.
But the deeper definition is more revealing.
A mineral is the product of both chemistry and architecture.
Its elements tell us what building materials nature had available.
Its crystal structure tells us how those materials were assembled.
Its physical properties tell us something about the bonds holding the structure together.
And its occurrence tells us something about the environment in which it formed.
That is why two substances made of carbon can behave as differently as graphite and diamond.
It is why natural diamond is a mineral while a laboratory-grown diamond, despite having essentially the same structure and chemistry, is technically a synthetic crystalline material.
And it is why a word as ordinary as mineral turns out to contain an extraordinary amount of science.
What Is a Mineral? FAQ
Is a lab-grown diamond a mineral?
Strictly speaking, no. A mineral must be naturally occurring. A lab-grown diamond is better described as a synthetic crystalline material. It can nevertheless have essentially the same chemical composition and crystal structure as natural diamond, so much of the same mineralogical science applies.
Is diamond a mineral?
Yes. Naturally occurring diamond is a mineral composed of carbon arranged in the diamond crystal structure.
Is ruby a mineral?
Ruby is a gem variety of the mineral corundum. Corundum is aluminum oxide, Al₂O₃, and chromium impurities are responsible for ruby's characteristic red color.
Is sapphire a mineral?
Sapphire is also a gem variety of corundum. Different trace elements can produce its various colors.
Is obsidian a mineral?
No. Obsidian is natural volcanic glass. Because it lacks the long-range crystalline structure required of a mineral, it is generally classified as a mineraloid.
Is opal a mineral?
Opal is commonly described as a mineraloid rather than a conventional crystalline mineral because it lacks the same degree of long-range atomic order.
Is ice a mineral?
Naturally occurring crystalline ice can qualify as a mineral because it is naturally occurring, solid, chemically defined and crystalline.
Is mercury a mineral?
Native mercury is generally excluded under the conventional mineral definition because mercury is liquid under ordinary surface conditions.
Are rocks minerals?
No. Rocks are generally aggregates of minerals and other geological materials. Granite, for example, commonly contains quartz, feldspar and mica.
What is the difference between a mineral and a crystal?
A mineral must occur naturally and satisfy mineralogical criteria. A crystal is defined by its ordered internal atomic structure and can be either natural or synthetic.